In the drilling of oil and gas wells, it is common to drive the drill bit by a downhole mud motor located at the end of a drill string. In particular, drilling fluid, generally referred to as drill mud, is circulated to drive the motor by positive hydraulic displacement or turbine action. The mud then passes through the ports in the drill bit and carries material loosed by the drill bit back to the surface through the annular space between the drill pipe and the resulting bore hole.
Bearing assemblies for wellbore drilling are mounted between the drill bit and the drill string to permit rotation of the drill bit. The drill bit is attached to a hollow drive shaft, also known as a mandrel that is located within a bearing housing. The mandrel is rotatably also known as a mandrel that is located within a bearing housing. The mandrel is rotatably driven by the mud motor while the bearing housing is fixed to the drill string and remains relatively stationary. In its position behind the drill bit, the bearing assembly is subject to significant radial and axial loading. Radial and thrust bearings are thus located along the bearing assembly to absorb radial and axial loads.
Lubrication between the rotator mandrel and stator housing may be achieved by oil or mud located in the annular space between those components. In the case of oil lubrication, an oil-sealed bearing chamber is formed by upper and lower seals. The seals are acted upon by downhole drilling fluid pressures, including pump pressures and hydrostatic pressures, resulting in higher pressures above the sealed bearing chamber as compared to below the sealed bearing chamber. Such pressure differential results in damage to the seals, leading ultimately to seal failure. To reduce the pressure differential, it is known to use a flow restrictor located above the sealed chamber in order to reduce the fluid flow in the annular passageway between the mandrel and housing.
The flow restrictor is usually quite brittle, and a radial bearing is typically provided above and below the flow restrictor to protect against bending forces. This necessitates two lubricated bearing chambers, where the upper bearing chamber must accommodate passages to allow drilling fluid flow between the mandrel and the housing in order to equalize pressure on either side of the upper bearing chamber. For example, U.S. Pat. No. 6,416,225 discloses a bearing assembly having a radial bearing assembly above the flow restrictor, with a separate sealed bearing chamber from the main sealed bearing chamber.
The mandrel component of the bearing assembly is also susceptible to damage by drilling loads, as well as by the severe shock and vibration incurred during drilling applications. In particular, the mandrel is engaged to the housing by a split ring, also called a saver ring. The split ring includes two semi-cylindrical halves having annular grooves in their inner surfaces. The machined grooves engage into annular recesses formed on the surface of the mandrel. During assembly, the halves of the split ring are fit over the mandrel. This form of assembly requires that the fit between the mandrel and the split ring to be somewhat loose. This loose fit permits some vibration between the mandrel and the split ring, thereby causing mandrel failure by cracking.
In some bearing assemblies, the mandrel includes a lock nut or a compression nut, which threads onto the mandrel and engages the housing, transmitting vertical loads between the housing and the mandrel. For example, U.S. Pat. No. 6,416,225 discloses a bearing assembly having a mandrel compression nut. While being an improvement over the use of a split ring, shock and vibration during the drilling process can still cause damage to the mandrel.
There is a need, therefore, for improved construction of a bearing assembly which provides for a longer operational life of the assembly over prior art constructions.